These lines are somewhat obscure, but maybe they can be understood in the context of the immediately following stanza:
But soon they grow again and leave their nest.
‘Oh!’ saith the Psalmist, ‘that I had a dove’s
Pinions to flee away, and be at rest!’
And who that recollects young years and loves,—
Though hoary now, and with a withering breast,
And palsied fancy, which no longer roves
Beyond its dimm’d eye’s sphere,—but would much rather
Sigh like his son, than cough like his grandfather?
So here Byron makes a contrast between the “youth, and vigour, beauty” of the hero, and the “hoary” “withering” age of—whom? Someone “that recollects young years and loves”, so this would seem to presage the old age of Don Juan, when his seductions will be at an end, and memory is all he has left of his youth.
So the reason why “those things … for an instant clip Enjoyment’s wings” is that youth and vigour bring with them a presentiment of age and death; but only for a moment.
Byron repeats this contrast between youth and age a couple of stanzas later:
But Juan was not meant to die so soon.
We left him in the focus of such glory
As may be won by favour of the moon
Or ladies’ fancies—rather transitory
Perhaps; but who would scorn the month of June,
Because December, with his breath so hoary,
Must come? Much rather should he court the ray,
To hoard up warmth against a wintry day.